feat: upgrade MiniMax from M2.5 to M2.7 and expand to more agents/categories

- Upgrade minimax-m2.5 → minimax-m2.7 (latest model) across all agents and categories
- Replace minimax-m2.5-free with minimax-m2.7-highspeed (optimized speed variant)
- Expand MiniMax fallback coverage to atlas, sisyphus-junior, writing, and unspecified-low
- Add isMiniMaxModel() detection function in types.ts for model family detection
- Update all tests (58 passing) and documentation

src/agents/types.test.ts

@@ -1,5 +1,5 @@
 import { describe, test, expect } from "bun:test";
-import { isGptModel, isGeminiModel, isGpt5_4Model } from "./types";
+import { isGptModel, isGeminiModel, isGpt5_4Model, isMiniMaxModel } from "./types";
 
 describe("isGpt5_4Model", () => {
   test("detects gpt-5.4 models", () => {
@@ -79,6 +79,28 @@ describe("isGptModel", () => {
   });
 });
 
+describe("isMiniMaxModel", () => {
+  test("detects minimax models with provider prefix", () => {
+    expect(isMiniMaxModel("opencode-go/minimax-m2.7")).toBe(true);
+    expect(isMiniMaxModel("opencode/minimax-m2.7-highspeed")).toBe(true);
+    expect(isMiniMaxModel("opencode-go/minimax-m2.5")).toBe(true);
+    expect(isMiniMaxModel("opencode/minimax-m2.5-free")).toBe(true);
+  });
+
+  test("detects minimax models without provider prefix", () => {
+    expect(isMiniMaxModel("minimax-m2.7")).toBe(true);
+    expect(isMiniMaxModel("minimax-m2.7-highspeed")).toBe(true);
+    expect(isMiniMaxModel("minimax-m2.5")).toBe(true);
+  });
+
+  test("does not match non-minimax models", () => {
+    expect(isMiniMaxModel("openai/gpt-5.4")).toBe(false);
+    expect(isMiniMaxModel("anthropic/claude-opus-4-6")).toBe(false);
+    expect(isMiniMaxModel("google/gemini-3.1-pro")).toBe(false);
+    expect(isMiniMaxModel("opencode-go/kimi-k2.5")).toBe(false);
+  });
+});
+
 describe("isGeminiModel", () => {
   test("#given google provider models #then returns true", () => {
     expect(isGeminiModel("google/gemini-3.1-pro")).toBe(true);